1. Field of the Invention
The present invention relates to a method for network layer handoff and more particularly to a method for network layer handoff over a wireless LAN.
2. Description of Related Art
Wireless local area network (WLAN), such as those which use the IEEE 802.11 protocol, have become increasingly popular and rapidly developing technology. When a user of WLAN moves from the coverage area of one access point (AP) to another, it is desirable that the network support rapid seamless handoff so that the user does not experience perceptible voice quality degradation. In many cases, especially in applications that use voice communication over a WLAN, an overlong period of time for network handoff often leads to obvious voice quality degradation or even the occurrence of “call-drop”.
The handoff over a WLAN generally involves two layers, i.e., link layer handoff denoting authentication and re-association with a new AP when a mobile node moves between the areas covered by different APs, and network layer handoff, which happens when a mobile node moves from one subnet to another subnet. In the latter case, the mobile node must acquire a valid IP address via schemes such as Dynamic Host Configuration Protocol (DHCP) after movement to another subnet. In general, network layer handoff, conveniently accomplished in several seconds, consumes much more time than link layer handoff. The ITU recommendation, G.114, allows a maximum time of 150 ms to 200 ms one-way latency for voice communication. Therefore, network layer handoff has been too high in latency time to be used for voice communication over a WLAN.
FIG. 1 is a schematic diagram showing the network layer handoff achieved based on a DHCP scheme over WLAN. In this figure, two wireless local area networks SN1 and SN2 are covered by two wireless access points AP1 and AP2, respectively. SN1 and SN2 are located in two different subnets, each of which has a DHCP server used for IP address assignment. AP1 and AP2 connect to switches SW1 and SW2, respectively. A router is connected with switches SW1 and SW2, and a personal computer PC1 is connected to the router via another switch SW3.
A mobile node MN1 within the first WLAN subnet SN1 establishes bi-directional communication with PC1 via the path of AP1-SW1-router-SW3. When mobile node MN1 moves from WLAN SN1 to another WLAN subnet SN2, a network layer handoff will occur in which mobile node MN1 obtains a new valid IP address from the DHCP server of the second subnet. At this point, mobile node MN1 communicates with PC1 through a new path of AP2-SW2-router-SW3. However, due to the duplicate address detection process required by the DHCP server, multiple instances of message interaction are required in the IP address acquisition process. Hence, the acquisition of a new IP address from the DHCP server leads to an overlong latency, generally on the order of seconds. This is unacceptable for applications such as voice communication.
In an article by Andrea G. Forte, Sangho Shin, and Henning Schulzrinne, “Improving Layer 3 Handoff Delay in IEEE 802.11 Wireless Networks”, published In Proceedings of the 2nd annual international workshop on wireless internet, Vol. 220, Boston, Mass., 2006, a method for reducing the latency of IP address acquisition via a DHCP server using a temporary IP address is introduced. In this article, a temporary IP address is acquired by a wireless mobile node at first for data transmission and, upon receipt of a formal IP address allocated from the DHCP server, the formal IP address is used for subsequent data transmission.
In this example, the acquisition of the temporary IP address is achieved by the sending of ten Address Resolution Protocol (ARP) requests by the wireless mobile node and testing ten successive IP addresses, e.g., the ten latest IP addresses used, to find possible IP addresses not in use. Such a test method is somewhat random and brings about an additional overhead of waiting during the timeout for the ARP response, more than about 100 ms, and thus cannot meet the strict latency requirement for voice transmission.